Unique Fields at Table Creation Time

Sintax:

[FieldName] type CONSTRAINT [constraint_name] UNIQUE

Example:

CREATE TABLE [Clients]
(
    ClientID int NOT NULL IDENTITY(1,1) CONSTRAINT [PK_Clients] PRIMARY KEY,
    ServiceCode varchar(50) NOT NULL CONSTRAINT [UQ_Clients_ServiceCode] UNIQUE
    ...
)

Example 2:

CREATE TABLE [Clients]
(
    ClientID int NOT NULL IDENTITY(1,1) CONSTRAINT [PK_Clients] PRIMARY KEY,
    ServiceCode varchar(50) NOT NULL,
    CONSTRAINT [UN_Clients_Unique] UNIQUE (ClientID, ServiceCode)
    ...
)